(1) Field of the Invention
The present invention relates to a method for the chromatographical analysis of anions in an aqueous solution, in which an electroconductivity detector is used.
(2) Description of the Related art
In the conventional chromatographical analysis of anions in an aqueous solution by using an electroconductivity detector, since the S/N ratio of the detector directly reflects the background of the electroconductivity of the eluant, the S/N ratio is reduced as the background is high. Accordingly, where a minute amount of an ion is measured, the S/N ratio should be improved; that is, the background should be reduced. Two different methods are known as means for attaining this object. According to one method, a cation exchange column having a hydrogen ion adsorbed therein is connected downstream from an analysis column for separating an anion in series thereto, the electroconductivity (background) possessed by an eluant is reduced by chemical conversion and, thus, the anion to be measured is detected at a high sensitivity [Small, Stevens and Bauman, Anal. Chem., 47, 1801 (1975)]. According to the other method, an anion is analyzed by using only a separating column without using a cation exchange column and an organic acid such as phthalic acid or tartaric acid is used as the eluant [Fritz, Gjerde and Becker, Anal. Chem., 52, 1519 (1980)].
The principle of the former method using a cation exchange column resides in the utilization of simple ion exchange. A solution containing several mM of sodium hydroxide or sodium hydrogencarbonate and sodium carbonate is used as the eluant, the anion to be measured is separated from the anion exchange resin in the analyzing column, the separated anion is introduced to the cation exchange resin in the cation exchange column having the hydrogen ion adsorbed therein, the sodium ion in the eluant is directly subjected to ion exchange with the restricted hydrogen ion electrostatically connected to the cation exchange resin, and the hydroxyl ion, which is a counter ion to the sodium ion in the eluant, and the carbonic acid ion are chemically converted to water and carbonic acid, respectively, whereby the electroconductivity possessed by the eluant can be controlled to a low level.
This method is expressed by the following chemical reaction formula: EQU OH.sup.- or CO.sub.3.sup.2- +H.sup.+ (bonded directly to the cation exchange resin).fwdarw.H.sub.2 O or H.sub.2 CO.sub.3
As another method utilizing this chemical principle, there can be mentioned a method using a cation exchange membrane instead of the cation exchange column [Hanaoka, Murayama, Muramoto, Matsuura and Nanba, J. Chromatogr., 239, 537 (1982)].
The principle of the latter method using an organic acid as the eluant resides in that since the difference of the electroconductivity between the ion in the eluant and the ion to be measured is reflected on the sensitivity, the electroconductivity possessed by the eluant is controlled to a low level by using as the eluant an organic acid having a low mobility, such as phthalic acid or tartaric acid, rather than an inorganic anion having a high mobility (a high electroconductivity).
As is apparent from the foregoing description, the known methods for the chromatographical analysis of anions by using an electroconductivity detector are based on limited chemical principles, and therefore, development of these methods has been inhibited. Under this background, it is desired to provide an analysis method based on a different chemical principle.